In these lectures we give an introduction to the basic elements of a
relativistic quantum field theory in the Lagrangian formulation,
involving scalar, vector, and
fermion fields, and indicate how to calculate amplitudes for
physical processes in perturbation theory with the help
of Feynman graphs.
The principle of local gauge invariance is explained in terms of the
well-known example of Quantum Electrodynamics (QED) with an Abelian
gauge symmetry and is then generalized to the case of non-Abelian
gauge invariance and applied to the formulation of
Quantum Chromodynamics (QCD). In the formulation of the electroweak theory
the gauge principle has to be supplemented by the concept of
spontaneous symmetry breaking with the help of the Higgs field
and by Yukawa interactions, for embedding massive particles in
a gauge-invariant way.
Excellent textbooks [1]} are available for further reading.
